Headwater catchments across the Western United States are critical environments to natural ecosystems and are a vital component of water resources to many communities and industries and may be strongly affected by changing climate and altered disturbance regimes. To better quantify how these systems may be affected by climate change and to develop more precise management strategies to mitigate the negative impacts, two studies were conducted at a plot scale in Lassen National Forest and at a regional scale across the Sierra Nevada’s. The first chapter addresses how forest management strategies such as thinning alter the soil moisture and sap flow dynamics during a severe drought. The reduced forest density across the thinned forest prolonged the initiation of the soil moisture recession, resulting in more available water later in the growing season. Furthermore, across the study area, a threshold response of sap flow velocity to low soil moisture levels was observed. The second chapter asserts a novel methodology to quantify the storage-flow relationship in headwater catchments across the Sierra Nevada’s using space based estimates of storage. A framework is then developed based on the storage-flow relationship and catchment hydrogeology to inform catchment susceptibility to decreasing flows at increasing drought severity. The results suggest that, at increasing drought severity, high storage capacity catchment flows are reduced at a greater rate than low storage capacity catchments.